Glaucoma describes a group of conditions which have in common progressive optic neuropathy with associated patterns of visual field loss. There are various known techniques to detect the presence of visual field loss due to glaucoma. One such known technique is the Moorfields Motion Displacement Test (MDT) which has been developed by a collaboration between Moorfields Eye Hospital, London, the UCL Institute of Ophthalmology and City University London, see Fitzke et al 1987, Verdon-Roe et al 2006a and Verdon-Roe et al 2006b (CE Mark Registered Number CE 2006/0012, Date: 14 Jul. 2006, Manufacturer AC Viswanathan).
The Moorfields MDT involves the presentation of a pattern of multiple vertical lines. The software design permits direct loading of researcher defined bitmaps into members of the object-oriented bitmap class without the need for recompilation, see Viswanathan 2000. The lines are each scaled in size by an estimate of retinal ganglion cell density, see Garway-Heath et al 2000a. This does not apply to centrally placed lines, which are not scaled by retinal ganglion cell density, but are sized to be resistant to the effects of optical blur so that the test can be performed without refractive (spectacle) correction, see Verdon-Roe 2006a. The line coordinates are selected by application of the Garway-Heath anatomical-functional map to reduce the number of locations tested in optic disc sectors that are over-represented in conventional perimetry, see Garway-Heath et al 2000b.
During the Moorfields MDT, the viewer is asked to maintain their gaze on a specific (fixation) target for the duration of the test. The lines are subjected one at a time to a brief period of horizontal oscillation at a frequency of approximately 5 Hz, see Verdon-Roe et al 2000, and duration of 200 ms per cycle, see Westcott et al 2000. Each period of line oscillation presents a stimulus to the visual field. The viewer is asked to indicate whenever they detect such a stimulus. The overall test comprises a sequence of such line presentations (stimuli), where each location is activated in turn, generally in accordance with some randomised order, and the view provides feedback for each stimulus that they can observe.
FIg. 1A is a schematic illustration of the results from an MDT for a spatial pattern of 52 locations to match the Humphrey 24-2 visual field test pattern of standard automated perimetry (SAP). The small circles indicate locations where the user successfully responded to the stimulus at that location, while the crosses indicate locations at which the viewer did not respond to the stimulus. The duration of the test is approximately 90 seconds to allow for a stimulus to be presented once at each test location.
However, it has been found in practice that the response of a viewer is not completely reliable (repeatable). In particular, there is approximately a 5% chance that a healthy observer will miss any given stimulus. For a test sequence comprising 52 locations (and one presentation per location), there is an expected rate of 2.6 false negative responses per test sequence (where a response is regarded as false negative if it incorrectly fails to be seen).
Accordingly, it is known in supra-threshold vision tests such as the Moorfields MDT to have a retest strategy, whereby the overall test sequence includes repetitions of the stimulus for at least some locations. FIG. 1B illustrates one such retest strategy, which involves repeating the stimulus once for each test location—i.e. each location has a first presentation and a second presentation (usually not consecutively, but rather interspersed with stimuli at other locations). If the first presentation and the second presentation both give the same result (either both positive or both negative), this result is then accepted as correct for that location.
On the other hand, if the first presentation and the second presentation give different results, then a third presentation is performed for this location. This leads to three presentations at the relevant location, and the final outcome is taken on a majority basis. In effect, this means that the result always corresponds to the outcome of the third presentation, since this necessarily matches one of the earlier presentations.
The approach of FIGS. 1A and 1B represent a supra-threshold test, in that it measures whether or not a subject exceeds a predetermined threshold. Such a supra-threshold test is used for screening, in that individuals who exceed the predetermined threshold are suspected of having visual field damage, and can therefore be provided with further testing. This is to be contrasted with a threshold test, which measures the limit of sensitivity for a given subject. In other words, the outcome of a supra-threshold test is a spatial distribution of binary values (pass/fail), whereas the outcome of a threshold test is a spatial distribution of numerical values corresponding to the threshold of visual sensitivity at each location.
In addition, the approach of FIG. 1B is referred to as a supra-threshold 2/3 test, in that it measures whether a subject can or cannot see the stimulus on a majority 2 out of 3 basis. In contrast, the approach of FIG. 1A is referred to as a supra-threshold 1/1 test, because the conclusion is based on the findings of just a single measurement.
It will be appreciated that the retest strategy of FIG. 1B involves testing each location at least twice—more particularly, usually about 10% of the locations are tested three times, while the remaining locations are tested twice. The number of false negatives with this approach is much lower than compared with the approach of FIG. 1A (reduced by a factor of about 10), so that the results are more reliable. However, having to provide repeat stimuli at a given location increases the duration of the overall test (by a factor of just over two, if scaling by the number of activations). This raises the practical costs of administering the test, since for a given set of equipment and medical support staff, the number of subjects that can be tested in a given time period is reduced, It is therefore desirable to be able to reduce the time taken for administering a supra-threshold vision test such as the Moorfields MDT, but without compromising the statistical reliability of the test.
Another use for a motion displacement test is to measure the visual sensitivity of a subject across the field of vision. For example, the movement (amplitude of oscillation) of the stimulus can be made smaller and smaller until a threshold is reached beyond which the movement is no longer discernable to the subject. Such sensitivity measurements can be used both for detecting the presence of disease or other damage to the visual field, and/or to measuring the ‘progression’ (deterioration) of such disease/damage. The more finely the amplitude of the stimulus can be controlled, the greater the accuracy of the results that can be obtained.
In most test environments, an LCD monitor is located approximately 0.3-0.4 m away from the subject, a distance which provides a comfortable focus and also allows the screen of the monitor to occupy the area of field of the subject that is required for assessing glaucoma. At this distance, the visual resolution of the subject may exceed the pixel resolution of the monitor, especially in the centre of the field of vision, where retinal ganglion cell density is greatest. This limits the motion displacement thresholds that can be measured with such equipment. The situation is worse with young people, who have relatively higher retinal ganglion cell density.
Another way of making a stimulus presentation more difficult to see is to reduce the stimulus energy (area* luminance) see Verdon-Roe 2006b associated with the presentation, for example by reducing the area or luminance intensity of the stimulus line. In practice, it is common to use lower stimulus intensities for younger people to compensate for their increased motion displacement sensitivity. However, in some cases the threshold measurement of the subject remains above the resolution of the monitor. This makes it more difficult to track the progression of visual sensitivity for a single subject over longer periods.
A further concern with visual tests such as an MDT is that a subject must maintain fixation during the test in order to obtain reliable results. In other words, the eye being tested must look at a fixed location during the screen (otherwise the locations being tested will shift relative to the visual field). Most existing visual field tests provide some form of fixation target, such as a central dot or cross, to help the subject maintain fixation. Nevertheless, loss of fixation remains a problem in obtaining reliable test results.